Stable Isotope Fractionation Research Articles

Evolution of Retisol impacted by artificial drainage: What can we learn from stable Fe isotope ratios?

Iron oxides are one of the most reactive mineral phases in soils. As a consequence, their transformations can considerably affect the dynamics of the adsorbed elements and of the associated soil ecosystem services. Understanding the dynamics of Fe oxides in soils is therefore a key issue for the evolution of soil and associated ecosystem services. A potentially powerful tool to study the transformations of Fe-oxides in soil is stable Fe isotopes. However, there are still important gaps in our knowledge of Fe isotope fractionations.In order to examine the Fe isotope fractionations related to each process occurring in soils, we focused on a drainage-influenced sequence of Retisols, a soil type characterized by clay translocation and subsequent degradation by redox processes inducing a strong spatial Fe segregation in contrasted soil volumes. We combined the isotopic approach at the scale of a bulk horizon and at the scale of the different soil volumes, with mineralogical analyses and mass balance calculations in order to investigate the consequences of the drainage on Fe isotope fractionation. We showed that while there were no Fe isotope fractionations at the profile scale, Fe isotopic signatures varied significantly among soil volumes (δ56Fe values from −0.49 ± 0.05 to 0.29 ± 0.06‰). These variations suggest that redox processes are the main mechanisms responsible for the Fe redistribution among the volumes, and particularly that Fe accumulation during Mn oxide precipitation makes a significant contribution to Fe isotopic fractionation, during these Retisol differentiation. In contrast, drainage-induced eluviation does not result in further Fe isotope fractionations in soil volumes in these Retisols. The isotopic signatures of the different mineral phases present in the volumes were calculated using the mass balance approach and suggest that the iron oxides (goethite, ferrihydrite) have δ56Fe values close to 0‰, while the clay minerals are enriched in heavy Fe isotopes and the Mn oxides in light Fe isotopes. This study provides insight into the dynamics of Fe minerals in hydromorphic soils and offers a new perspective on stable Fe isotope fractionation in soils.

Stable isotope fractionation of cadmium in the soil-rice-human continuum

Consumption of rice (Oryza sativa) grain is a major pathway by which humans are exposed to Cd, especially in non-smoking Asian populations. Although the stable isotope signatures of Cd offer a potential tool for tracing its sources, little is known about the isotopic fractionation of Cd across the entire soil-rice-human continuum. Cadmium isotope ratios were determined in field soils, rice grain, and human urine collected from two Cd-contaminated regions in southern China. Additionally, Cd isotopic fractionation in rice plants was investigated using two transgenic plants differing in Cd uptake and accumulation. Analysis of isotope ratios revealed a preferential enrichment of the heavy Cd isotopes from soil to rice grain (δ114/110Cdgrain-soil = +0.40‰) and from grain to urine (δ114/110Cdurine-grain = +0.40‰) in both regions. The first increase was mainly caused by partitioning between the soil solid phase and the soil solution, with heavier Cd preferentially enriching in the soil solution. Within the rice plant, we identified multiple processes that alter the isotope ratio, but the net effect throughout the plant was comparatively small. Cd fractionation in humans is presumably due to the preferential enrichment of heavier Cd isotopes by metal transporters DMT1 and ZIP8 (responsible for the absorption of Cd into body from the foods). These findings provide important insights into the Cd isotopic fractionation through the soil-rice-human continuum and are helpful for tracing the sources of Cd.

Cadmium transfer in contaminated soil-rice systems: Insights from solid-state speciation analysis and stable isotope fractionation.

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (δ114/110Cd=-0.37 to-0.39%) compared to the plants that grew on non-flooded soils (δ114/110Cd=-0.45 to-0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl2 root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Δ114/110Cdshoot-root 0.16-0.19‰). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Δ114/110Cdshoot-root-0.02 to 0.08‰). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd-S complexes in the vacuole. Cd-S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems.

Mass-independent and mass-dependent Cr isotopic composition of the Rumuruti (R) chondrites: Implications for their origin and planet formation

Chromium (Cr) isotopes play an important role in cosmochemistry and planetary science because they are powerful tools for dating (53Mn–53Cr short-lived chronometry), tracing (54Cr nucleosynthetic anomalies) the origins of the materials, and studying the processes involved in volatile element fractionation and planetary differentiation (Cr stable isotopic fractionation). To use Cr isotopes for these purposes, it is essential to precisely know the compositions of the various chondritic reservoirs. However, the Cr isotope composition of Rumuruti (R) chondrites remains unknown. Here, we report high-precision mass-independent (average 2SE uncertainty of ∼0.02 and ∼0.06 for ε53Cr and ε54Cr, respectively; ε indicates 10,000 deviation) and mass-dependent (uncertainty of average 0.03‰ for δ53Cr; δ indicates 1000 deviation) Cr isotope data for 12 bulk R chondrites of petrologic types 3–6 (including R chondrite breccias), and one R chondrite-like clast (MS-CH) in the Almahata Sitta polymict ureilite. All the R chondrites show homogeneous bulk ε54Cr values, −0.06 ± 0.08 (2SD), similar only to those of the Earth–Moon system and enstatite chondrites. This first ε54Cr dataset for R chondrites provides significant addition to the ε54Cr–Δ17O diagram, and positions them as a potential endmember for planetary precursors. The R chondrites possess a higher 55Mn/52Cr of 0.68 ± 0.04 relative to most of carbonaceous chondrites and higher ε53Cr values 0.23 ± 0.05 (2SD) relative to most of chondrite groups. This likely results from the lower chondrule abundance in R chondrites compared to that of ordinary and enstatite chondrites. The stable Cr isotope composition of R chondrites is homogeneous with a δ53Cr = −0.12 ± 0.03‰ (2SD). Combined with previous data of other groups of chondrites, we show that the stable Cr isotopic composition of all the chondrites is homogeneous with δ53Cr of −0.12 ± 0.04‰ (2SD, N = 42) and is independent of the petrologic type and redox conditions. The lack of mass-dependent fractionation between all groups of chondrites suggests that the average chondrite δ53Cr value is also representative for the initial composition of all differentiated planets in the Solar System. Finally, the MS–CH clast in Almahata Sitta has a Cr isotopic composition (ε53Cr = 0.18 ± 0.04, ε54Cr = −0.16 ± 0.07, and δ53Cr = −0.11 ± 0.05‰) that is consistent (within error) with it being an R chondrite-like clast.

The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts

Accurate knowledge of the composition of Earth’s major chemical reservoirs is fundamental for constraining all modern geochemical cycles. Basaltic rocks provide a direct way of sampling the composition of Earth’s inaccessible interior. Here, we present the first comprehensive neodymium (Nd) stable isotope analyses for a global compilation of mid-ocean ridge, ocean island, continental intraplate and island arc basalts using a double-spike technique. In these primitive magma compositions magmatic differentiation has no resolvable effect on δ146/144Nd. Mid-ocean ridge basalts possess an extremely homogenous δ146/144Nd with an average composition of δ146/144Nd = −0.025 ± 0.013‰ (±2 s.d.; n = 33). Ocean island and continental intraplate magmas possess more variable compositions (δ146/144Nd = 62 ppm) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (δ146/144Nd = 61 ppm) reflect the complex interplay between source composition, degree of melting and slab-fluid inputs. Variations are uncorrelated with indicators of magmatic differentiation or slab-fluid addition, rather increasing δ146/144Nd with slab depth is attributed to a higher proportion of metasomatized sub-arc mantle in the melting region. A partial melting model for Nd stable isotopes has been constructed using NdO force constants calculated using the Born-Lande approximation. Melting of typical mantle peridotite will induce no resolvable fractionations of Nd stable isotopes (Δ146/144Ndmelt-mantle < 0.003‰ at 1200 °C). The lack of fractionation upon partial melting means primitive magmatic rocks can be used to calculate the average composition of the bulk silicate Earth (BSE), which is δ146/144Nd = −0.024 ± 0.031‰ (±2 s.d.; n = 80). This BSE composition is indistinguishable at the 95 % confidence level from that of chondritic meteorites, the building blocks of Earth. Therefore, sequestration of significant quantities of Nd into the sulfide matte did not occur, this combined with recent experimental evidence for no Sm-Nd fractionation means the sulfide matte cannot be considered a plausible solution for the 142Nd/144Nd offset between the Earth and chondrites. Despite resolvable variations in δ146/144Nd from the canonical value being widespread in terrestrial materials, they are not large enough to generate the difference in radiogenic Nd isotope ratios between the BSE and chondrites.

Mercury stable isotope fractionation during gaseous elemental mercury adsorption onto coal fly ash particles: Experimental and field observations

Mercury (Hg) stable isotopes have a great potential to track coal combustion Hg emissions, but mass-dependent fractionation (MDF) during Hg adsorption onto fly ash particles could significantly alter isotope signatures of emitted Hg species. The detailed processes causing this MDF, however, are not well understood. Here, we simulated how isotopes fractionate during gaseous Hg0 adsorption onto fly ash at different times and temperatures. Kinetic MDF that preferably transfers light Hg isotopes to fly ash dominated Hg0 adsorption processes. The magnitude of MDF during Hg0 adsorption was invariable in the time-series experiment but increased significantly with increasing temperature in the temperature-series experiment. The external mass transfer and chemisorption are suggested to be the controlling processes for isotopic fractionation. Relative to diffusion-driven Hg0 adsorption, chemisorption is suggested to be a more important Hg0 adsorption step causing MDF, especially at high temperatures. The chemisorption involves Hg redox change from Hg0 to HgII and is likely enhanced with increasing temperature (50–180 °C). The proposed kinetic MDF model reveals that MDF in modern coal-fired power plants is likely driven by temperature-induced redox processes during Hg0 adsorption, and has great implications for developing MDF models in coal-fired boilers and tracing coal combustion Hg emissions.

Stable Fe isotope fractionation during dissimilatory Fe(III) reduction by a thermoacidophile in acidic hydrothermal environments

Dissimilatory iron reduction (DIR) plays an essential role in biogeochemical Fe cycling in anoxic environments. At near-neutral pH, in both biotic and abiotic systems, aqueous Fe(II) (Fe(II)aq) interacts with reactive ferric (hydr)oxides via electron transfer and atom exchange that is catalyzed by large amounts of sorbed Fe(II). This may result in substantial Fe isotope exchange, which, at equilibrium, produces up to a ∼4‰ 56Fe/54Fe fractionation between coexisting oxide/hydroxide and Fe(II)aq, depending on mineralogy. The role of biology in such systems has been interpreted to lie in the production of Fe(II) rather than a specific “vital” effect, such as enzymatic and kinetic processes. Under acidic abiotic conditions, however, the lack of sorbed Fe(II) generates little Fe isotope exchange, and, by extension, it has been expected that little exchange would occur during DIR at low pH if sorbed Fe(II) is the key component for catalyzing isotopic exchange.In this study, we explored the extent and mechanism of Fe isotope exchange between Fe(II)aq and ferric hydroxides (ferrihydrite and goethite), including determination of the 56Fe/54Fe fractionations during DIR by Acidianus strain DS80 at pH ∼ 3.0 and 80 °C, over 19 days of incubation. Significant Fe(III) reduction occurred for both minerals along with large changes in Fe isotope compositions for Fe(II)aq, indicating Fe isotope exchange. Solid-phase extractions using HCl confirmed a lack of sorbed Fe(II), which suggests that a mechanism other than sorption is required to catalyze Fe isotope exchange during DIR at low pH. Reactive Fe(III) (Fe(III)reac) extracted from the mineral surface allowed for the calculation of the Fe pools that underwent isotopic exchange. A total of ∼20% of goethite and ∼60% of ferrihydrite underwent isotopic exchange over 19 days. For goethite from biotic experiments, we calculate a Fe(III)reac-Fe(II)aq fractionation factor of 1.57 ± 0.52‰, which is larger than the abiotic equilibrium fractionation factor (∼0.73‰ at 80 °C). This result is consistent with previous work on DIR of goethite at neutral pH, where a fractionation factor larger than equilibrium was interpreted to reflect an isotopically distinct “distorted surface layer” of goethite produced during exchange with Fe(II)aq. In contrast to goethite, the difference between the Fe(III)reac-Fe(II)aq fractionation factor for ferrihydrite from our biotic reactors (2.91 ± 0.40‰) and the abiotic equilibrium fractionation factor (∼2.28‰ at 80 °C, under silica-free conditions) is smaller.Ultimately, the contrast in the extent of Fe isotope exchange between biotic and abiotic experiments emphasizes the importance of biology in promoting Fe isotope exchange in acidic systems. We speculate that the unique role of biology at low pH in catalyzing Fe isotope exchange, not seen in equivalent abiotic systems, must lie in the transport of electrons to the ferric hydroxide surface that produces Fe(II) atoms in situ. This suggests that isotopic exchange occurs on an atom-by-atom basis as Fe(III) is reduced to Fe(II), followed by the release of Fe(II) into solution. This study demonstrates that significant variations in Fe isotope compositions may be uniquely produced in acidic environments where microbial Fe cycling occurs via DIR, compared to minor isotopic variations observed previously in acidic abiotic systems.

Reductive debromination by sponge-associated anaerobic bacteria coupled to carbon isotope fractionation

Marine sponges contain diverse brominated compounds as secondary metabolites and the sponge habitat appears to enrich for a population of anaerobic dehalogenating bacteria. Hence, there is interest in understanding how these natural and anthropogenic compounds are degraded in the marine environment. Compound specific isotope analysis (CSIA) is a useful tool to monitor and to quantify the degradation and fate of aquatic pollutants. The objective of this study was to evaluate whether reductive dehalogenation of brominated phenols by sponge-associated bacteria, including Desulfoluna spongiiphila, can be monitored by CSIA. Debromination of 2,6-dibromophenol to phenol by sponge-associated cultures resulted in measurable stable carbon isotope fractionation. All sponge-associated cultures showed similar isotopic enrichment factors (ε). The ε values for two independent sponge-derived dehalogenating cultures were −3.1 ± 1.5‰, and −3.0 ± 0.3‰, and that of sponge associated sediment cultures −2.0 ± 0.3‰. Thus, we demonstrate that reductive debromination of 2,6-dibromophenol resulted in measurable carbon isotope fractionation and that CSIA can be used to assess reductive debromination and to monitor and estimate in vivo dehalogenation in a sponge animal.

Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems

Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the CaO and MgO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the CaO difference between Grt and Cpx (ΔCaOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCaOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The MgO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.

Stable nitrogen and oxygen isotope fractionation during precipitation of nitrate salt from saturated solutions.

Nitrate is an oxyanion similar to CO3 - and thus should undergo stable N and O isotope fractionation during dissolution or precipitation. This process should dominate abiotic soil nitrate processes in hyperarid regions of Earth and possibly Mars. The N and O isotope fractionations during the precipitation of nitrate salt from saturated solutions at ~20°C were determined by two methods: rapid precipitation by antisolvent crystallization and slow uninhibited precipitation in a desiccator. In the antisolvent crystallization procedure, increasing volumes of acetone were added to samples of saturated sodium and strontium nitrate solutions to instantaneously precipitate nitrate salt. In the slow procedure (requiring weeks), slow evaporative water loss drove the process. There was little difference between the two procedures. Using a Rayleigh model, the calculated N fractionation (15 εproduct-residual ) ranged from 1.69‰ to 2.77‰, whereas for O, the 18 εproduct-residual values were between 1.27‰ and 4.61‰. The N isotope fractionation between NO3 - and the metal solid is similar to that between C in dissolved CO3 -2 and carbonates. We found that O versus N isotope plots of soil nitrate in a cold/dry Antarctic chronosequence had slopes similar to those from the experiments, revealing abiotic transport. In the Atacama Desert, where the soil nitrates are a mix of biological and tropospheric nitrate, there is an inverse relationship between soil N and O isotopes. These two relationships were compared with the isotope composition of nitrate from Martian meteorite EETA79001. While the N and O isotope composition of the Martian nitrate is remarkably similar to that of the present Atacama Desert, the interpretation of the slope of the O versus N isotopes remains ambiguous due to the limited number of samples. Additional NO3 samples from Martian meteorites are needed to address the question of abiotic versus biotic alteration of NO3 - on Mars.

Algal biomarkers as a proxy for pCO2: Constraints from late quaternary sapropels in the eastern Mediterranean

Records of carbon dioxide concentrations (partial pressure expressed as pCO2) over Earth’s history provide trends that are critical to understand our changing world. To better constrain pCO2 estimations, here we test organic pCO2 proxies against the direct measurements of pCO2 recorded in ice cores. Based on the concept of stable carbon isotopic fractionation due to photosynthetic CO2 fixation (Ɛp), we use the stable carbon isotopic composition (δ13C) of the recently proposed biomarker phytol (from all photoautotrophs), as well as the conventionally used alkenone biomarkers (from specific species) for comparison, to reconstruct pCO2 over several Quaternary sapropel formation periods (S1, S3, S4, and S5) in the eastern Mediterranean Sea. The reconstructed pCO2 values are within error of the ice core values but consistently exceed the ice core values by ca. 100 µatm. This offset corresponds with atmospheric disequilibrium of present day CO2[aq] concentrations in the Mediterranean Sea from global pCO2, equivalent to ca. 100 µatm, although pCO2 estimates derived from individual horizons within each sapropel do not covary with the ice core values. This may possibly be due to greater variability in local CO2[aq] concentration changes in the Mediterranean, as compared with the global average pCO2, or possibly due to biases in the proxy, such as variable growth rate or carbon-concentrating mechanisms. Thus, the offset is likely a combination of physiological or environmental factors. Nevertheless, our results demonstrate that alkenone- and phytol-based pCO2 proxies yield statistically similar estimations (P-value = 0.02, Pearson’s r-value = 0.56), and yield reasonable absolute estimations although with relatively large uncertainties (±100 µatm).

Oxygen isotope fractionation between water and the aqueous hydroxide ion

The stable oxygen isotope fractionation factor between water and the aqueous hydroxide ion is a fundamental geochemical parameter widely used in the Earth Sciences and other fields. Studies from the 1960s reported α(H2O(l)-OH-(aq)), the fractionation factor between liquid H2O and aqueous OH−, theoretically as 1.046 and ∼1.048 (46‰ and 48‰ at 15 °C and 25 °C) and experimentally as 1.045 (45‰) at 15 °C. These, or similar values have been used in the literature for almost 60 years. Here I present quantum-chemical (QC) calculations, showing that the above theoretical values are fundamentally wrong as they pertain to free OH− (incorrectly assumed equivalent to aqueous OH−) and ignore intermolecular vibrational modes in solution. As a result, the theoretical values from the 1960s are off by a factor of ∼2 (when expressed in ‰), suggesting that the experimental value is also wrong. QC computations of OH−-water clusters with up to n=22 water molecules demonstrate that hydrogen bonding in solution significantly affects the fundamental vibrational modes associated with OH− and substantially reduces the oxygen isotope fractionation between water and OH-(aq), compared to the fractionation between water and free OH−. The most accurate QC methods tested here yield values for the fractionation factor between H2O and OH− in water clusters with n=7 to 22 water molecules of ∼1.019 to ∼1.024 (19 to 24‰) at 25 °C based on the harmonic approximation. Estimated effects due to anharmonicity (from numerically demanding computations) could add uncertainties of up to ∼3‰ to these values.

Carbonization of Carbonates and Fractionation of Stable Carbon Isotopes in a Dynamic Slip Zone

Data obtained on carbonates and metaterrigenous rocks of the Devonian complex of the Sultan-Uizdag Range, northwestern Uzbekistan, which are carbonized in local deformation zones, are used in a generalized interpretation of the genesis of carbonaceous matter by means of decomposition of carbonates under the effect of seismogenic movements. The release of free carbon is facilitated by synchronous Fe–Mg metasomatism and by the reducing conditions in the presence of hydrogen. Based on literature data on experimental modeling of carbonization processes at high-speed shear deformations of carbonates in a hydrogen medium, a possible scenario is considered for the generation of hydrogen on the surfaces of quartz dislocations in the presence of water in metaterrigenous rocks in association with carbonates. The variation of carbon isotope composition over the volume of the carbonate and terrigenous rocks is extremely heterogeneous, with the 13C isotope enriched in the fracture zones and 12C enriched in the near-fracture volumes of decompression-damaged rocks. It is hypothesized that deformations can affect the fractionation of stable carbon isotopes. With regard for literature data, it is demonstrated that isotope exchange can proceed between the free carbon and carbonate carbon. Various aspects are discussed of the influence of mineral-phase and structural–textural transformations of carbonate rocks at their thermomechanical decomposition under shearing on the geomechanical behavior of the fracture, manifestations of mechanical instability in it, and causes of frictional weakening.

The Mechanisms of the Interaction of Stable Isotopes with Biological Objects in the Presence of an Uncompensated Neutron in Chemical Bonds

The physical regularities that lead to isotope fractionation and to the accumulation of certain isotope forms in the intercellular and intracellular space at various levels of biological objects were studied. A new hypothesis of fractionation of stable isotopes in biological objects through the neutron effect was discussed. The following feature is noted: it is very likely that the development of isotope shock in living beings is observed mainly in the presence of covalent bonds between atoms with an uncompensated neutron. One possible explanation for this phenomenon can be associated with a change in the physical parameters of the following events: the interactions of the magnetic moments of valence electrons with the magnetic moments of atomic nuclei and the interactions of magnetic moments of atomic nuclei, which result in a change in the distance between atomic nuclei.

Differential behavior of tungsten stable isotopes during sorption to Fe versus Mn oxyhydroxides at low ionic strength

Tungsten is an emerging contaminant of interest with serious health implications whose environmental transport and fate have scarcely been studied. Sorption to mineral particles is likely the primary process controlling tungsten's mobility in oxidizing soils and aquifers, but the few papers published thus far have not yet resolved the relationships between aqueous speciation, substrate phases, and sorption complexes at conditions relevant to these settings. In many cases, these relationships are best investigated with techniques such as X-ray spectroscopy, but the low concentrations of tungsten in typical contaminated settings limit the applicability of that approach. Here, we lay initial groundwork for eventual use of the stable isotope geochemistry of tungsten to infer sorption mechanisms at field-relevant conditions, because metal stable isotope systematics are sometimes very sensitive to speciation and surface complexation geometry. We report the first tungsten stable isotope fractionations for soil-relevant conditions using simple batch experiments, in which tungsten interacted with either birnessite (Mn oxyhydroxide) or ferrihydrite (Fe oxyhydroxide) nanoparticles, at pH 5 or 8 and very low ionic strength.In 120-h experiments, in which proportions of sorbate and sorbent were varied, lighter isotopes of W preferentially sorbed on ferrihydrite. On a plot of δ183/182W against % W sorbed, parallel lines fit the pH 8 data from ferrihydrite experiments well, with a best fit Δ183/182Wdissolved-sorbed of +0.39‰ (R2 = 0.95), indicating an equilibrium fractionation (reversible sorption in a closed system). Parallel, linear trends also fit the data from ferrihydrite experiments at pH 5 well, with a best-fit fractionation of +0.32‰ (R2 = 0.93). When error bars are propagated rigorously, these two Δ183/182Wdissolved-sorbed values are indistinguishable, indicating that fractionation is insensitive to pH in the W-ferrihydrite system over the relevant range of pH and suggesting that sorption mechanism may also not vary with pH. In every birnessite experiment, lighter isotopes preferentially sorbed, but fractionation magnitude varied systematically with fraction of W sorbed, with larger fractionations at larger fraction of total W sorbed. At pH 8, the range in Δ183/182Wdissolved-sorbed was +0.12 to +0.40‰, with an average of 0.31‰, and at pH 5, the range was +0.29 to +0.67‰, and the average was +0.47 ± 0.10‰. Isotope behavior and sorption mechanisms appear to be different for birnessite and more complicated compared to ferrihydrite; neither parallel lines (equilibrium trends) nor Rayleigh trends obviously fit all the data well. This observation led us to consider the possibility of a kinetic isotope effect superimposed on equilibrium fractionation, where the expressed mixture of kinetic and equilibrium fractionation varies with both surface loading and time.To explore further, we conducted experiments at pH 5 with fixed W/birnessite proportions and durations up to 504h. Results indicate that the amount of W sorbed to birnessite continues to increase gradually for at least 3 weeks. The magnitude of isotope fractionation also appears to increase gradually over time, from ~0.3‰ at ≤48 h. to ~0.6‰ at >100 h. We use multiple lines of indirect evidence to hypothesize that W initially sorbs directly to birnessite surfaces, but gradually a polymeric W-O surface precipitate phase assembles and grows, as observed in previous publications for W sorption on other substrates. In future work, we will attempt to confirm this hypothesis with X-ray spectroscopic analysis. Once the relationship between isotopic fractionation and sorption mechanism(s) are constrained, use of tungsten isotopes to probe sorption mechanisms at field-relevant concentrations of tungsten may be possible. In addition, inclusion of sorption-driven isotope systematics in reactive transport models may be useful in tracing W mobility in contaminated soils and sediments.

Numerical simulation of stable isotope fractionation with density changes in regional-scale confined geothermal aquifers

Stable isotope compositions in groundwater are widely used to understand hydrological processes. To infer changes in isotope composition in the subsurface due to interacting processes such as dispersion, chemical reactions, and phase changes, numerical simulations were employed to relate the isotope composition in the groundwater to that in the recharge water. It was early identified in the laboratory that the variation of temperature and pressure can lead to the isotope fractionation in water. This indicated that for geothermal aquifers with fluid density contrasts due to the variation of temperature and pressure, the isotope fractionation among varying-density fluids can occur. However, this effect has not been considered in regional-scale geothermal aquifers previously. This study numerically investigated the relationship between density-contrasts and the distribution of stable isotope in confined aquifers with temperatures up to 180 °C and depths of <3000 m. We found that the magnitude of δ18O fractionation relating to density contrasts can reach 0.02‰ per meter in a 100-m-thick homogeneous aquifer. This magnitude is comparable to the δ18O variation with elevation (~0.25‰ per 100 m) and temperature (0.23‰ per °C) on the land surface. Hence, not accounting for isotope fractionation among varying-density fluids can lead to errors in interpreting the recharge area and recharge temperature. An empirical model was derived that relates the magnitude of δ18O fractionation to the density contrast in geothermal aquifers. This model can help correct errors in estimates of the recharge altitude and temperature due to temperature and pressure variations changes in the aquifers.

Inorganic carbon uptake strategies in coralline algae: Plasticity across evolutionary lineages under ocean acidification and warming

Dissolved inorganic carbon (DIC) assimilation is essential to the reef-building capacity of crustose coralline algae (CCA). Little is known, however, about the DIC uptake strategies and their potential plasticity under ongoing ocean acidification (OA) and warming. The persistence of CCA lineages throughout historical oscillations of pCO2 and temperature suggests that evolutionary history may play a role in selecting for adaptive traits. We evaluated the effects of pCO2 and temperature on the plasticity of DIC uptake strategies and associated energetic consequences in reef-building CCA from different evolutionary lineages. We simulated past, present, moderate (IPCC RCP 6.0) and high pCO2 (RCP 8.5) and present and high (RCP 8.5) temperature conditions and quantified stable carbon isotope fractionation (13ε), organic carbon content, growth and photochemical efficiency. All investigated CCA species possess CO2-concentrating mechanisms (CCMs) and assimilate CO2 via diffusion to varying degrees. Under OA and warming, CCA either increased or maintained CCM capacity, which was associated with overall neutral effects on metabolic performance. More basal taxa, Sporolithales and Hapalidiales, had greater capacity for diffusive CO2 use than Corallinales. We suggest that CCMs are an adaptation that supports a robust carbon physiology and are likely responsible for the endurance of CCA in historically changing oceans.

Magnesium isotope fractionation reflects plant response to magnesium deficiency in magnesium uptake and allocation: a greenhouse study with wheat

AimsMagnesium (Mg) deficiency is detrimental to plant growth. However, how plants respond to Mg deficiency via regulation of Mg uptake and allocation is yet not fully understood. In this study, we tested whether Mg isotope compositions (δ26Mg) associated with Mg mass balance of the plants could be used as an indicator to trace Mg uptake and subsequent translocation processes under sufficient and low-Mg supply conditions. We aimed at using stable isotope fractionation as a novel proxy for nutrient uptake and cycling in plants.MethodsWe grew wheat plants (Triticum aestivum) in a greenhouse under control (1 mM Mg) and low-Mg supply (0.05 mM Mg) conditions, respectively. The Mg concentrations and isotope compositions in roots, stems, leaves and spikes/grains at different growth stages were analyzed.ResultsWheat plants were systematically enriched in heavy Mg isotopes relative to the nutrient solution regardless of Mg supply conditions. With crop growth, the δ26Mg of the whole plants, as well as each plant organ, gradually shifted towards higher values in the control. However, the δ26Mg value of the whole plants in the low-Mg supply did not vary significantly. In addition, the wheat stems and spikes showed continuous enrichment of lighter Mg isotopes in the low-Mg supply than those in the control.ConclusionsAs reflected from Mg isotope compositions, the Mg supply in the growth media could affect the Mg uptake and subsequent translocation processes in plants. Changes in δ26Mg indicated that wheat plants likely regulated their Mg uptake strategy by switching between active and passive pathways during their life cycle. When Mg supply was low, a more negative δ26Mg value of the spikes suggested a potentially enhanced remobilization of Mg from leaves to spikes. Our results showed that Mg stable isotopes can provide new insights into plants’ response to nutrient shortage.

The dolomite problem: evidence from 3D modeling, XRD and geochemical data of Zechstein reefs (Upper Permian, Germany)

Three dimensional modeling of the limestone and dolomite distribution in an Upper Permian (Zechstein) stromatolite-bryozoan reef, ca. 500m in diameter and 35m thick (77 drill cores, 172 point data), shows that dolomite occurs as laterally and vertically discontinuous intervals. The prevailing mineral phases are near stoichiometric dolomite and Mg free calcite, respectively (370 XRD and 341 XRF analyses). Both ∂ 13 C and ∂ 18 O (526 analyses) show a spread of ca. 10‰ and covary with the mineralogy, and the heaviest dolomite and calcite ∂ 13 C differ by ca. 1,5‰. Diagenetic modifications caused by flowing meteoric fluids could account for the observed “inverted J” trend of stable and the radiogenic signature of 8 7 Sr/ 86 Sr (23 analyses), but neither vertical nor horizontal gradients occur in the modeled reef. Because dolomite geometries are incompatible with those predicted by fluid flow models, and the limestone-dolomite difference in ∂ 13 C overlaps estimates of isotope fractionation associated with Mg content, the studied dolomite was a depositional Very High Mg Calcite recrystallized to dolomite in a semi-closed diagenetic system rather than a Low Mg Calcite transformed by a dolomitization process. The isotope pattern suggests biogenic fractionation and/or loss of heavy stable and light Sr isotopes during diagenesis.

Carbon nanotubes accelerate acetoclastic methanogenesis: From pure cultures to anaerobic soils

Direct interspecies electron transfer (DIET) between electricigens and methanogens has been shown to favour CO2 reduction to produce biomethane. Furthermore, DIET is accelerated by conductive materials. However, whether conductive materials can promote other methanogenic pathways is unclear due to a lack of detailed experimental data and the poor mechanistic studies. Here, we hypothesized that conductive carbon nanotubes (CNTs) stimulate acetoclastic methanogenesis independently of electricigens in pure cultures of Methanosarcina spp. and anaerobic wetland soil. We found a significant increase in the methane production rate during the growth phase, e.g. from 0.169 mM to 0.241 mM after addition of CNTs on the 3rd day. CNTs did not increase the abundance of electromicroorganisms or the electron transfer rate in anaerobic soils, using via microbial diversity and electrochemical analysis. 13C–CH3COOH labelling, stable carbon isotope fractionation and the CH3F inhibitor of acetoclastic methanogenesis were used to distinguish methanogenic pathways. CNTs mainly accelerated acetoclastic methanogenesis rather than CO2 reduction in both pure cultures and anaerobic soils. Furthermore, the presence of CNTs slightly alleviate the inhibition of CH3F on acetoclastic methanogenesis during the pure culture of Methanosarcina barkeri and Methanosarcina mazei with the production of more than 0.3 mM methane. CNTs closely attached to the cell surface were observed by transmission electron microscopy. Proteome analysis revealed a stimulation of protein synthesis with about twice the improvement involved in –COOH oxidation and electron transfer. Overall, our findings demonstrate that conducting CNTs favor methane production and that the mechanism involved is acetoclastic methanogenesis via acetate dismutation, at least partly, rather than classical CO2 reduction.